Large volumes of natural gas, primarily methane, are often contained in coal seams. After a wellbore has been drilled through a coal seam, it is desirable to be able to extract the gas, typically to be used as a resource, but also for safety reasons (to degassify the coal seam) if the coal will be subsequently mined.
Coal deposits tend to exhibit relatively low permeability, which complicates the production of natural gas from coal seams.
Various techniques are known to increase the permeability of a coal deposit and thus stimulate the production of methane from a coal seam. Typically in these techniques, the coal seam adjacent to a wellbore is fractured to help create a direct path from pockets of methane within the coal seam to the wellbore. Fracturing typically involves the introduction into the wellbore of a fluid under pressure.
One such fracturing method is hydraulic fracturing by the injection of liquids. However, hydraulic fracturing is expensive and also creates the potential problem of unwanted fluids in the coal seam or in the wellbore.
Another method, such as that taught in U.S. Pat. No. 5,014,788, which issued to Puri on May 14, 1991, involves the use of a gas, such as carbon dioxide (CO2), injected into the wellbore at pressure. When the pressure of the CO2 within the wellbore has reached a given level, a surface valve is opened to release the CO2 rapidly. The process is repeated several times. This pressure cycling, with rapid depressurization, creates stress fractures within the coal seam, allowing methane within the coal seam to escape into the wellbore. However, the need to introduce a gas into the wellbore, such as CO2, increases the cost of methane extraction. Moreover, the need to pressurize and depressurize the entire wellbore is expensive and requires substantial time, given that a wellbore can extend far below ground. Further, in a deep well, due to the large volume of CO2 that must be released for the pressure cycling described above, pressure cycling can be relatively slow.
Therefore, it would be desirable to be able to increase the permeability of a coal seam without the need to introduce a fluid, either gas or liquid. It would be desirable to increase the permeability of a coal seam more cost-effectively and/or more efficiently. It would also be desirable to increase the permeability of a coal seam, if a fluid is introduced, without needing to pressurize and depressurize the entire wellbore. It would also be desirable to utilize pressure cycling techniques to increase the permeability of coal seams.
The permeability in the vicinity of a wellbore of formations other than coal seams may also be relatively low, either due to the natural state of the formation or due to damage caused during drilling the wellbore or well operations after the wellbore has been drilled.
For example, a formation may become damaged during drilling by the introduction into the wellbore of a drilling fluid under pressure, which drilling fluid may accumulate sand, rock or clay particles as it circulates through the wellbore. These particles may tend to clog or plug a formation adjacent to the wellbore.
Therefore, it would be desirable to be able to increase the permeability of any subject formation adjacent to a wellbore without the need to introduce a fluid, either gas or liquid. It would be desirable to increase the permeability of a subject formation more cost-effectively and/or more efficiently. It would also be desirable to increase the permeability of a subject formation, if a fluid is introduced, without needing to pressurize and depressurize the entire wellbore. It would also be desirable to utilize pressure cycling techniques to increase the permeability of a subject formation.